As shown in FIG. 10, a magnet assembly 400 for a dental magnetic attachment has a structure comprising a soft-magnetic stainless steel disc yoke 414 and a non-magnetic stainless steel seal ring 415 concentrically arranged in an opening of a soft-magnetic stainless steel cup-shaped yoke 401; welding being made between the disc yoke 414 and the seal ring 415 and between the seal ring 415 and the cup-shaped yoke 401 along their entire boundaries to seal a permanent magnet 402. As shown in FIG. 9, this magnet assembly 400 is embedded in a denture floor 420 of denture 430, and held by a root cap 422 embedded in an alveolus 421 by magnetic attraction between the magnet assembly 400 and a soft-magnetic keeper 423 attached to the root cap 422. The magnet assembly 400 should meet requirements such as no harm to a human body, a long period of chemical stability, large attraction, etc.
For example, as described in JP 5-95965 A, the magnet assembly 400 for a dental magnetic attachment is produced by placing a permanent magnet 402 (for example, a neodymium magnet) in a recess of the cup-shaped yoke 401, inserting a disc-shaped seal member 413 composed of the disc yoke 414 and the seal ring 415 into an opening of the cup-shaped yoke 401, welding an abutment of the cup-shaped yoke 401 and the seal ring 415, and an abutment of the seal ring 415 and the disc yoke 414 for sealing, and then making the welded portions flat by polishing or grinding. When this magnet assembly 400 is attracted to the keeper 423, the seal ring 415 of austenitic stainless steel existing in a peripheral portion of the disc-shaped seal member 413 shuts part of a magnetic path, so that a magnetic flux of the permanent magnet 402 can efficiently flow through a magnetic circuit constituted by the keeper 423, the cup-shaped yoke 401 and the disc yoke 414, resulting in large attraction.
The disc-shaped seal member 413 is generally produced by inserting a round rod of ferritic stainless steel into an austenitic stainless steel pipe, conducting drawing to form a round clad rod comprising a peripheral portion of austenitic stainless steel and a center portion of ferritic stainless steel, and transversely cutting it. Conventionally used for the seal ring 415 is austenitic stainless steel obtained by forming an austenite phase by dissolving nickel. However, a nickel-containing magnet assembly for a magnetic attachment cannot be used for patients having nickel allergy. Thus desired for a dental magnetic attachment is a magnet assembly of nickel-free stainless steel.
As nickel-free austenitic stainless steel, for example, JP 2012-92413 A discloses nitrogen-dissolved austenitic stainless steel obtained by dissolving nitrogen in a stainless steel composition, and proposes a method for producing nitrogen-dissolved austenitic stainless steel by heating ferritic stainless steel at 1100-1250° C. in a nitrogen atmosphere at about atmospheric pressure (partial pressure of nitrogen: 80-86.7 kPa), so that the ferritic stainless steel surface absorbs nitrogen.
However, because the nitrogen-dissolved austenitic stainless steel is transformed to have a mixed phase of ferrite and Cr nitride at a high temperature of about 700° C. or higher and at atmospheric pressure, this material cannot be heated for abutment welding or extrusion to form a pipe for the seal ring, or the annealing of a pipe to remove strain.